Fein vacuum hold-down works fine
After fiddling with screws and clamps as hold-down devices on my alpha 120, and spending more time fixing material to the Shopbot than cutting, I decided to try a Fein vacuum and Shopbot's 4-zone sbp file. It works - very well.
I'm using 2-inch pipe throughout with each of the 4 sections connected to a 2-inch manifold via a shutoff valve (with a 5th section available for 60x60 baltic birch).
On top of the vacuum board, for parts that I do repetitively, I use 1/8-inch plywood as a vacuum mask. The advantage of using a vacuum mask is that I can cut each part completely free without messing with tabs. I am going to try thicker material for the vacuum mask. The 1/8-inch plywood works, but it is more fragile than I would like and it leaves enough debris that I have to watch carefully for loose splinters that tend to lift the material being cut. Perhaps 1/4 or 3/8-inch mdf would work better and still be cost-effective. The disadvantage of using a vacuum mask is that it costs $8-$10 per mask, takes time to fabricate, and must be stored or discarded after use; however, the cost is much less than the cost of having the Shopbot stand idle while I messed with screws and clamps to fix material to the spoil board.
For the one-off parts, I screw the material that I'm cutting to a sheet of 1/2-inch mdf spoil-board. The 1/2-inch mdf gets sucked down flat, which makes the material sheet flat also. The advantage in this method is that I can mount material to spoil boards without halting production on the shopbot. The disadvantage is that I have to use tabs to hold the cut parts together. Since the spoil boards can be reused, cost is not much of a factor.
In any case, using the Fein vacuum has increased production. (I had assumed that a small shop-vac type vacuum wouldn't be able to do the job, and, since I've extremely limited on available power, I knew that I couldn't use any vacuum system that was capable of sucking parts down through a spoil board.)
Many thanks to Eric who mentioned in a recent post that most shops doing sheet work suck parts down through a spoil board. Until he wrote that, or words to that effect, I didn't have a clue on how easily the vacuum could work. After spending a few minutes on the web and letting Google find info on "CNC router spoil board", the light finally came on and the many posts on this forum about various vacuum systems made sense.
